ABC News examined the attacks in which the Tracker identified three or more people killed -- which include the shooter in some cases.

There was at least one shooting every month and in three cases, two in a single day.

With six incidents, Texas had the most mass killings, including a biker shootout in Waco that left nine dead and campsite attack in Anderson County that left six dead.

Just last week, Robert Dear was accused of killing three people at a Colorado Planned Parenthood clinic. In June, Dylann Roof allegedly walked into a South Carolina church and shot and killed nine people. In that case, the FBI said its background-check system failed, mistakenly allowing the 21-year-old to purchase the gun he allegedly used in the rampage.

While these shootings gained national attention, there were many others that did not have the same notoriety.

Many of the shooting deaths involved domestic incidents, some were suspected gang activity and others were people caught in the crossfire. These shootings took place across the country and impacted people of all races, cultures and socio-economic classes.

All of the incidents involved firearms, but some of the victims died by other means. Federal data on mass killings was not available for 2015.

Following Wednesday's attack, President Obama called on the federal government to take steps that would "make it make it harder, not impossible, but harder to individuals to get access to weapon."

"I do think that as the investigation moves forward it’s going to be important for all of us, including our legislatures, to see what we can do to ensure that when individuals decide that they want to do somebody harm, we’ll make it a little harder for them to do it," he said.

Speaker of the House Paul Ryan (R-Wisconsin) said that lawmakers should not rush to pass legislation until the facts are known, but did call for reform to mental health laws to deal with the issue of gun deaths in the U.S.

"What we are trying to do is make sure our response actually addresses the problems without infringing upon the rights of law-abiding citizens, Ryan said Thursday to CBS "This Morning". "A lot of these people who are getting guns who are mentally unstable, who should not be getting guns and this is a gap in our laws that we feel needs to be filled.